An organic EL device is a spontaneously luminous device which features higher brightness and higher legibility than those of liquid crystal devices enabling vivid display to be attained and has, therefore, been vigorously studied.
In 1987, C. W. Tang et al. of Eastman Kodak Company have developed a device of a layer-laminated structure comprising various kinds of materials to bear individual roles, and have put an organic EL device using organic materials into a practical use. The above organic EL device is constituted by laminating layers of a fluorescent body capable of transporting electrons and of an organic material capable of transporting holes. Upon injecting both electric charges into the layer of the fluorescent body to emit light, the device is capable of attaining a brightness of as high as 1000 cd/m2 or more with a voltage of not higher than 10 V (see, for example, a patent document 1 and a patent document 2).
So far, very many improvements have been made to put the organic EL device to practical use. For example, the organic EL device has been widely known having a structure comprising an anode, a hole injection layer, a hole-transporting layer, a luminous layer, an electron-transporting layer, an electron injection layer and a cathode which are arranged in this order on a substrate more finely dividing their roles than ever before. The device of this kind is achieving a high efficiency and a high durability.
To further improve the luminous efficiency, attempts have been made to utilize triplet excitons and study has been forwarded to utilize a phosphorescent luminous compound.
In the organic EL device, the electric charges injected from the two electrodes recombine together in the luminous layer to emit light. Here, however, what is important is how efficiently to hand both electric charges, i.e., holes and electrons, over to the luminous layer. Upon improving the electron injection property, improving the mobility thereof and, therefore, improving the probability of recombination of the holes and the electrons and, further, confining the excitons formed in the luminous layer, it is allowed to attain a high luminous efficiency. Namely, the electron-transporting material plays an important role. Therefore, it has been desired to provide an electron-transporting material that has a high electron injection property, a large electron migration rate, a high hole-blocking property and a large durability against the holes.
As for the life of the device, further, the heat resistance and amorphousness of the material also serve as important factors. The material having small heat resistance is subject to be thermally decomposed even at a low temperature due to the heat generated when the device is driven, and is deteriorated. The material having low amorphousness permits the thin film thereof to be crystallized in short periods of time and, therefore, the device to be deteriorated. Therefore, the material to be used must have large heat resistance and good amorphousness.
Tris(8-hydroxyquinoline)aluminum (hereinafter abbreviated as Alq) which is a representative luminous material has also been generally used as an electron-transporting material having, however, a hole-blocking power which is far from satisfactory.
A method of inserting a hole-blocking layer is one of the measures for preventing the holes from partly passing through the luminous layer to improve the probability of recombination of the electric charge in the luminous layer. As a hole-blocking material, there have heretofore been proposed trizaole derivatives (see, for example, a patent document 3). There have, further, been known a bathocuproin (hereinafter abbreviated as BCP) and a mixed ligand complex of aluminum [aluminum (III) bis(2-methyl-8-quinolinato)-4-phenyl phenolate (hereinafter abbreviated as BAlq).
There has, further, been proposed a 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter abbreviated as TAZ) as a material having excellent hole-blocking power (see, for example, the patent document 3).
The TAZ has a work function of as large as 6.6 eV and a large hole-blocking power, and is used for forming an electron-transporting hole-blocking layer that is laminated on the cathode side of a fluorescent luminous layer or a phosphorescent luminous layer prepared by vacuum evaporation or by coating and, therefore, contributes to improving the efficiency of the organic EL devices.
Because of a serious problem of low electron-transporting capability, however, the TAZ had to be used in combination with an electron-transporting material having a higher electron-transporting capability to fabricate the organic EL devices.
The BCP, on the other hand, has a work function of as large as 6.7 eV and a large hole-blocking power but a glass transition point (Tg) of as low as 83° C. In the form of a thin film, therefore, the BCP lacks stability and still cannot be said to be sufficiently working as the hole-blocking layer.
Either material still lacks stability when it is formed into a film or lacks the function for blocking the holes to a sufficient degree. In order to improve characteristics of the organic EL devices, therefore, it has been desired to provide an organic compound that excels in electron injection/transporting capability and in hole-blocking power, and features high stability in the form of a thin film.